Orexins A and B (or hypocretins 1 and 2) are hypothalamus neuropeptides of 33 and 28 amino acids, respectively, recently identified as endogenous ligands of two seven-domain transmembrane receptors, known as orexin 1 and orexin 2 receptors (Sakurai T., Cell, Vol. 92, 573-585, 1998; De Lecea L., Proc. Natl. Acad. Sci., Vol. 95, 322-327, 1998).
The orexin 2 receptor has the property of recognizing the two forms of orexin A and B equivalently. In contrast, the orexin 1 receptor, which shows 64% homology with the orexin 2 receptor, is more selective and binds orexin A ten times better than orexin B (Sakurai T., Cell, Vol. 92, 573-585, 1998).
Via these receptors, the orexins control various central and peripheral functions, especially taking of food and drink, certain cardiovascular endochrine functions and the awake/sleep cycle (Sakurai T., Regulatory Peptides, Vol. 85, 25-30, 1999).